1. Field of the Invention
The present invention relates generally to method for preparing particles of pyrithione salts, and more specifically to methods of preparing particles of pyrithione salts having non-spherical or non-platelet forms, particularly needle or rod forms. The present invention also relates to products made with the non-spherical or non-platelet particles of pyrithione salts.
2. Description of the Related Art
Polyvalent metal salts of pyrithione (also known as 1-hydroxy-2-pyridinethione; 2-pyridinethiol-1-oxide; 2-pyridinethione; 2-mercaptopyridine-N-oxide; pyridinethione; and pyridinethione-N-oxide) are known to be effective biocidal agents, and are widely used as fungicides and bactericides in paints and personal care products such as anti-dandruff shampoos. The polyvalent metal salts of pyrithione are only sparingly soluble in water and include magnesium pyrithione, barium pyrithione, bismuth pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione. The most widely used divalent pyrithione salts are zinc pyrithione and copper pyrithione.
Zinc and copper pyrithione are useful as antimicrobial agents active against gram-positive and negative bacteria, fungi, and yeasts. Zinc pyrithione is used as an antidandruff component in shampoos, while technical suspensions of zinc pyrithione and/or copper pyrithione are used as preservatives in paints and polymers. Synthesis of polyvalent pyrithione salts are described in U.S. Pat. No. 2,809,971 to Berstein et al. Other patents disclosing similar compounds and processes for making them include U.S. Pat. Nos. 2,786,847; 3,589,999; 3,590,035; 3,773,770.
Known methods for producing insoluble polyvalent salts of pyrithione result in platelet-shaped large particles having an average size greater than 2 micrometers (.mu.m). These particles are either used directly, or converted into smaller spherical particles. Such small spherical particles of pyrithione salts are usually generated by a separate mechanical manipulation step (e.g., grinding or crushing) on larger particles or crystals that are made by conventional processes. For example, European Patent Application No. 70046 describes preparation of zinc pyrithione using organic solvents. This process results in production of large crystals of zinc pyrithione. A separate, optional grinding step is used to grind the large crystals and produce zinc pyrithione particles of smaller size. Small spherical particles of zinc pyrithione more easily form suspensions and provide a larger surface area for enhanced biocidal activity. However, grinding larger particles to produce smaller particles, as described in the prior art, generally results in substantial loss of useful product and are costly in terms of equipment, time, and energy required. In addition, alternative forms of pyrithione particles, such as rods, needles, or other shapes, cannot be produced by grinding.
Alternative forms of insoluble polyvalent pyrithione particles are often desirable because they offer certain physical properties that are not inherent in platelet or spherical forms. Elongated particles of pyrithione salts, such as rods, needles, ellipsoids, and the like offer advantages such as a large and/or flat surface area for use in shampoos, soaps and paints. In addition, elongated particles are more easily isolable by conventional filtration methods than the platelet particles of the prior art.
U.S. Pat. No. 5,540,860 to Hosseini et al. discloses a general method for producing several forms of copper pyrithione particles, such as rods, spheres, needles, platelets, and combinations thereof at processing temperatures of about 70.degree. C. or greater by using a variety of surfactants including POLYTERGENT 2A-1L, POLYTERGENT SLF-18, and TRITON X-100 surfactants. However, a general method of producing non-platelet or non-spherical forms of pyrithione salts is not disclosed. The present inventors have found that such a general method requires the use of lower temperatures and/or dispersants such as salts of polymerized alkyl naphthalene sulfonic acids. Neither of these parameters are disclosed in the '860 patent to Hosseini.
Thus, what is needed in the art is a general method of generating non-spherical or non-platelet particles of pyrithione salts in a single step production process at low temperatures. The present invention is believed to be an answer to that need.